Various devices are known for atomising a liquid. For example, the documents EP-A-0 923 957 and EP-A-1 005 916, both in the name of the present Applicant describe a liquid droplet spray device. A brief description of the liquid droplet spray device known from these documents, which are hereby incorporated by reference, is given here while referring to FIG. 1.
In this particular embodiment spray device 1 consists of a housing formed of a superposition of a first, or a top substrate 5 and a second, or a bottom substrate 6 in-between which a chamber or a space 2 is formed for containing a liquid substance 3 and thus providing a capillary filling and compression chamber. Top substrate 5 contains outlet means consisting of cavity or cavities 7 which can partly constitute space 2, outlet nozzles 9 and output channels 10 connecting these nozzles to space 2.
Liquid substance 3 enters spray device 1 by way of, e.g., a very low pressure, e.g., around a few millibar or slightly negative pressure, or capillary action. This can be achieved for example by way of at least one input tube or needle 4 through which the liquid substance may be supplied from an external reservoir (not shown) into spray device 1. Spray device 1 further comprises a vibrating element 8, e.g. a piezoelectric element to cause vibration of liquid substance 3 in space 2.
The method of manufacturing this device is carried out by using technology known from the field of semiconductors. Thus, top and bottom substrates may be manufactured in a similar manner e.g. by etching a silicon wafer in a suitable manner, e.g. by wet or dry etching and by using one or more masks or by micro-machining Pyrex wafers. The substrates 5 and 6 are attached to each other, preferably by appropriate bonding technique, such as anodic bonding, so as to form and enclose space 2.
These prior art documents further describe techniques allowing for output channels with a straight, non-tapered profile. This provides for a precisely defined pressure drop, droplet size and flow behaviour across output channel 10 for aqueous solutions and suspensions whereas the relatively smooth surface is suited for medications carrying small solid particles, e.g. from less than 1 to approx 2 μm, in suspensions. But output channels with a straight, non-tapered profile are also suitable for more viscous liquids, such as ambient fragrances which depending on the fragrance concentration however would normally tend to wet the surface of top substrate 5 and therefore might inhibit effective dispensing of such liquids.
The same effect can be obtained proportionally with larger dimensions, e.g. with nozzles of 10 μm or larger for example for personal perfume or for functional liquid dispensing applications or in a practical variation of the cited prior art of the applicant by simply using the vertical plasma etching micro-machining method to produce an output channel whose cross-section is divided into two or more identical sub-channels to allow for an even finer control of pressure drop, droplet size and flow behaviour across said channel 10. The cross section of the vertical channel or channel section can be of a suitable geometrical form, e.g. circular, triangular or a suitable geometrical shape such as a cross when the channel consists of several identical sub-channels. The cross section of the cavities 7 can also be of suitable geometrical form or combination of forms.
FIG. 2a shows a schematic detailed view of the first, or top substrate of this prior art liquid droplet spray device. The top substrate is shown upside down with respect to FIG. 1 in a further practical variation of the cited prior art which has already been shown with an inversion of the bottom substrate, thus further reducing dead space. As can be seen, top substrate 5 comprises the cavities 7, output channels 10 and outlet nozzles 9. The top surface of the substrate-delimiting cavity 7 forms a membrane section in substrate 5.
The surface of this membrane section is much larger than the actual nozzle surface, so that it is possible to provide a very large number of outlet nozzles 9 on the membrane surface in order to eject more droplets simultaneously. As already mentioned in the cited prior art, it is obvious that cavities 7 are not necessarily tapered but can be straight according to the manufacturing process chosen. FIG. 2b shows a close-up view of a part of FIG. 2a in which it can be seen that the output channels 10 and outlet nozzles 9 may be readily placed according to the specific conditions.
The diameter of a droplet depends among other factors on the nozzle hole size “d” for a given frequency of the vibration of the liquid substance and the inlet pressure. In this prior art device where a frequency of around 250 kHz is used, the mean droplet diameter has been found to be around 5 μm, the diameter of the hole of nozzle 9 is around 7 μm and the inlet pressure is a few millibars. One such a droplet thus contains a quantity of around 67 femtolitres (10−15 l) so that the number of nozzles may be determined as a function of the amount to be ejected.
The document EP 1 149 602 shows an embodiment where the top substrate may be micromachined in such a way as to provide recessed areas around the output nozzles such as shown in FIG. 4 of this document. Thus, the actual nozzle outlet protrudes from the main surface of the top substrate and contributes to the monodispersive nature of the ejected spray by providing minimum stiction surface for the liquid around the output nozzles. Alternatively if the total area constituted by the membrane section in substrate 5, meaning the total top surface of the substrate-delimiting cavity 7 is recessed, all output nozzles will protrude.
A further liquid droplet spray device is known from the document WO-A-00/06388. This device also has a first substrate provided with a piezo-electric vibrating element, and a second substrate provided with outlet means.
Both substrates enclose a chamber for containing a liquid substance, in a manner similar to the above-described prior art. The outlet means are manufactured in such a way that here too recessed areas are created around the nozzle outlets so that the outlet nozzles protrude from the main surface of the second substrate so as to reduce stiction.
However, these devices use expensive manufacturing techniques such as DRIE (deep Reactive Ion Etching) or plasma etching) and many manufacturing steps on a very large surface of silicon, resulting in a comparatively expensive device.
Further, it is known that the droplet diameter varies with certain physico-chemical properties of the liquid such as surface tension and viscosity. It is therefore important as shown in the cited prior art to be able to adapt the physical and electrical device parameters (frequency and amplitude) according to the liquid to be expelled and the desired droplet characteristics.
The applicant has now found that although the prior art device generally functions satisfactorily, the construction of this device has limits if it needs to be manufactured in a cheap manner, such as when used as a personal or ambient perfume dispenser instead of a very precise medication dispenser, thereby still ensuring sufficient rigidity and precision when manufacturing the nozzle outlet means and therefore complying with formal, informal or introductory specifications required by environmental and health related institutions.
Furthermore, when such a device is used to expel liquid substances of high viscosity such as perfume or some functional liquids, there is a much larger problem of retention of the liquid when being expelled from the nozzle outlet means leading to wetting and an uncontrollable droplet size, because portions of the droplets to be expelled may stick to the outer surface of the nozzles and create a thin liquid film there which will interfere with following droplets trying to detach from the nozzles. A higher power is then needed to actually cause the droplet to detach from the nozzle outlet, and if the power were not high enough, small droplets would then be released as a part and stay behind as a satellite droplet due to the retention with the film on the top surface surrounding the nozzle outlets caused by the stiction of the expelled liquid. For some liquids surface wetting due to retention forces being higher than dispensing forces will go beyond creating satellites, it may simply prevent droplet generation. This problem will be compounded if nozzles are set closer to each other for reasons of cost hence size reduction. Surface capillary forces will tend to create a liquid film connecting all nozzles.
For liquids requiring to be expelled in larger droplets and consequently being dispensed by nozzle outlets with larger diameters, the effect of retention on the surface might turn into a straightforward leakage, even if the device is passive and especially if it is held upside down. Up to a certain point, the use of flexible foil airless bags might prevent leakage, but as of a certain diameter of the outlet nozzle, this also becomes ineffective.
It is, therefore, an object of the present invention to provide a nozzle body for a liquid droplet spray device as well as a liquid droplet spray device that overcomes the above-mentioned inconveniences and that can be efficiently used for high viscous liquids such as perfumes or other non-aqueous solvent based liquids.
It is another object of the present invention to provide such a nozzle body and device that is simple, reliable and inexpensive to manufacture, small in size and low in energy consumption and cost, and as such suitable as a personal or ambient fragrance and functional liquid dispenser.